-Drug resistance and tumor recurrence continue to be important challenges affecting the therapeutic outcomes for breast cancer patients. Recent studies suggest that cancer stem cells (CSCs), a sub-population of the tumor with `stem cell-like' self-renewal properties, contribute to disease recurrence. Current therapies do not effectively eradicate CSCs. Our studies show that targeting cytotoxic drugs specifically to CSCs reduces tumor recurrence in a mouse model of breast cancer. We now propose to build on this exciting finding by delivering the cytotoxic agent in a nanoparticle formulation that is directed to CSCs using a high affinity targeting ligand recently developed in one of our laboratories. Nanoparticulate delivery systems that are in the size range of ca. 100 nm show enticing tumor accumulation and intra-tumoral penetration properties. A major limitation of current nanomedicines in this small size regime, however, is their inability to be formulated to have high drug load levels or sustained drug release, let alone both. With the support of an R21 award, our team has discovered a novel strategy that uses a hydrolytically labile silicate ester of paclitaxel (PTX), namely [PTX-Si(OR)3], as a prodrug construct. The greater hydrophobicity of these silicates, when used in conjunction with flash nanoprecipitation (FNP) as the means for nanoparticle (NP) synthesis, uniquely allows the preparation of stable, small, block copolymer- protected NPs containing up to an unprecedented 60-75 wt% of cargo, here the prodrug. In an independent thrust, we have successfully developed a single chain variable fragment (scFv) that recognizes CD133, a unique marker presented on the surface of CSCs. We now propose to marry these two exciting inventions by developing CD133-targeted (using our new scFv), 100 nm NPs that contain high percentages of (pro)drug cargo and that show a prolonged duration of payload release. We expect these formulations to have greatly improved therapeutic efficacy. Our Specific Aims are to: Aim 1) Develop CSC-targeted, silicate prodrug-loaded NPs that have high drug loading and adjustable drug regeneration profiles Aim 2) Determine the in vivo safety and efficacy of CSC-targeted, silicate prodrug-loaded NPs. Innovations will be enabled by partnering our novel silicate prodrug strategy with FNP technology that, together, will give small NPs that have high drug loading (>50 wt%) and prolonged timelines for regeneration of free PTX (a goal is ?1 week for release of half of the NP payload). Use of the novel CSC targeting ligand substantially enhances the approach. In addition to developing CSC-targeted NPs as highly effective anticancer therapeutics, we will advance a fuller understanding of the fundamental relationship between the physicochemical properties of NPs and their therapeutic performance.